This invention relates generally to the field of monitoring of analytes in the body. More particularly, this invention relates to a non-invasive method of increasing the permeability of skin and other membranes through ultrasound or a combination of ultrasound and chemical enhancers for selectively enhancing outward flux of analytes from the body for monitoring thereof.
The stratum corneum is chiefly responsible for the well known barrier properties of skin. Thus, it is this layer that presents the greatest barrier to transdermal flux of drugs or other molecules into the body and of analytes out of the body. The stratum corneum, the outer horny layer of the skin, is a complex structure of compact keratinized cell remnants separated by lipid domains. Compared to the oral or gastric mucosa, the stratum corneum is much less permeable to molecules either external or internal to the body. The stratum corneum is formed from keratinocytes, which comprise the majority of epidermis cells, that lose their nuclei and become corneocytes. These dead cells then form the stratum corneum, which has a thickness of only about 10-20 .mu.m and, as noted above, is a very resistant waterproof membrane that protects the body from invasion by exterior substances and the outward migration of fluids and dissolved molecules. The stratum corneum is continuously renewed by shedding of corneum cells during desquamination and the formation of new corneum cells by the keratinization process.
The flux of a drug or analyte across the skin can be increased by changing either the resistance (the diffusion coefficient) or the driving force (the gradient for diffusion). Flux may be enhanced by the use of so-called penetration or chemical enhancers. Chemical enhancers are well known in the art and a more detailed description will follow.
Other methods of increasing the permeability of skin to drugs have been described, such as ultrasound or iontophoresis. Iontophoresis involves the application of an external electric field and topical delivery of an ionized form of drug or an unionized drug carried with the water flux associated with ion transport (electro-osmosis). While permeation enhancement with iontophoresis has been effective, control of drug delivery and irreversible skin damage are problems associated with the technique.
Ultrasound has also been used to enhance permeability of the skin and synthetic membranes to drugs and other molecules. Ultrasound has been defined as mechanical pressure waves with frequencies above 20 kHz, H. Lutz et al., Manual of Ultrasound 3-12 (1984). Ultrasound is generated by vibrating a piezoelectric crystal or other electromechanical element by passing an alternating current through the material, R. Brucks et al., 6 Pharm. Res. 697 (1989). The use of ultrasound to increase the permeability of the skin to drug molecules has been termed sonophoresis or phonophoresis.
U.S. Pat. Nos. 4,309,989 to Fahim describes topical application of medications in a coupling agent for the treatment of Herpes virus infections and demidox mite infestations. The medications are massaged into the affected area by ultrasound to cause the medication to penetrate the skin. U.S. Pat. No. 4,372,296 to Fahim similarly describes topical application of zinc sulfate and ascorbic acid in a coupling agent for treatment of acne.
U.S. Pat. No. 4,767,402 to Kost et al. discloses a method for enhancing and controlling infusion of molecules having a low rate of permeability through skin using ultrasound in the frequency range of between 20 kHz and 10 MHz, and in the intensity range of between 0 and 3 W/cm.sub.2. The molecules are either incorporated in a coupling agent or, alternatively, applied through a transdermal patch. Kost et al. further teach that the parameters of time, frequency, and power can be optimized to suit individual situations and differences in permeability of various molecules and of various skins. U.S. Pat. No. 4,780,212 to Kost et al. teaches use time, intensity, and frequency control to regulate the permeability of molecules through polymer and biological membranes. Further, the choice of solvents and media containing the molecules also affects permeation of the molecules through the membranes. Transbuccal drug delivery with ultrasound has also been disclosed, U.S. Pat. No. 4,948,587 to Kost et al. There is no suggestion however that the techniques disclosed can be utilized for the recovery of analytes.
U.S. Pat. No. 5,115,805 to Bommannan et al. discloses the use of specific frequencies (i.e. &gt;10 MHz) of ultrasound to enhance the rate of permeation of drugs through human skin into the body. Frequencies above 10 MHz gave improved penetration of the skin above that described earlier. It is alleged that chemical penetration enhancers and/or iontophoresis can also be used in connection with the ultrasound treatment to enhance delivery of drugs through the skin into the body.
U.S. Pat. No. 5,016,615 to Driller et al. involves local application of a medication by implanting a drug-containing receptacle adjacent to a body tissue to be treated and then applying ultrasound to drive the drug out of the receptacle and into the body tissue. This method has the disadvantage of requiring surgical implantation of the drug receptacle and a noninvasive technique is preferred. U.S. Pat. No. 4,821,740 to Tachibana et al. discloses a kit for providing external medicines that includes a drug-containing layer and an ultrasonic oscillator for releasing the drugs for uptake through the surface of the skin. In U.S. Pat. No. 5,007,438 to Tachibana et al. is described an application kit in which a layer of medication and an ultrasound transducer are disposed within an enclosure. The transducer may be battery powered. Ultrasound causes the medication to move from the device to the skin and then the ultrasound energy can be varied to control the rate of administration through the skin.
Other references teaching use of ultrasound to deliver drugs through the skin include D. Bommannan et al., 9 Pharmaceutical Res. 559 (1992); D. Bommannan et al., 9 Pharmaceutical Res. 1043 (1992); K. Tachibana, 9 Pharmaceutical Res. 952 (1992); P. Tyle & P. Agrawala, 6 Pharmaceutical Res. 355 (1989); H. Benson et al., 8 Pharmaceutical Res. 1991); D. Levy et al., 83 J. Clin. Invest. 2074 (1989).
Permeation through the stratum corneum can occur by (a) intracellular penetration, (b) intercellular penetration, and (c) transappendageal penetration, especially through the sebaceous pathway of the pilosebaceous apparatus and the aqueous pathway of the salty sweat glands. The utility of ultrasound in enhancing the permeability of the stratum corneum and, consequently, increasing transdermal flux rate is thought to derive from thermal and mechanical alteration of biological tissues. The parameters of ultrasound that are manipulable to improve or control penetration include frequency, intensity, and time of exposure. All three of these parameters may be modulated simultaneously in a complex fashion to increase the effect or efficiency of the ultrasound as it relates to enhancing the transdermal molecular flux rate either into or out of the human body. Other factors also play a part, for example the composition and structure of the membrane through which molecules are to be transported, the physical and chemical characteristics of the medium in which the molecules are suspended, and the nature of the molecules themselves. Since ultrasound is rapidly attenuated in air, a coupling agent, preferably one having lowest realizable absorption coefficient that is non-staining, nonirritating, and slow drying, may be needed to efficiently transfer the ultrasonic energy from the ultrasound transducer into the skin. When a chemical enhancer fluid or anti-irritant or both are employed, they may function as the coupling agent. For example, glycerin used as an anti-irritant may also function as a coupling agent. If needed, additional components may be added to the enhancer fluid to increase the efficiency of ultrasonic transduction. In general, ultrasound exposure times for permeation through human skin have been less than 60 minutes, preferably less than 10 minutes. It has been suggested that the maximum limit of exposure should be determined by monitoring skin temperature. However, monitoring of skin surface temperature would not necessarily monitor events such as rupture of cell membranes by mechanical shear forces which could occur at low temperatures with short duration, high intensity ultrasound. The exposure may be either continuous or pulsed to reduce heating of biological membranes. Average intensities have been in the range of 0.01-5 W/cm.sup.2 and are selected to be high enough to achieve the desired result and low enough to avoid significant elevation of skin temperature. Frequencies have varied from 20 kHz to 50 MHz, preferably 5-30 MHz. The depth of penetration of ultrasonic energy into living soft tissue is inversely proportional to the frequency, thus high frequencies have been suggested to improve drug penetration through the skin by concentrating the effect in the outermost skin layer, the stratum corneum. Various refractive and/or reflective ultrasonic focusing systems may also be employed to concentrate the ultrasonic energy in the desired tissue independent of the fundamental frequency. When appropriate phase conditions are met, resonance of the system can be induced to favor higher frequency harmonic components of the fundamental ultrasonic energy, causing local zones of ultrasonic energy at 2, 3, 4 or more times the fundamental frequency.
Although it has been acknowledged that enhancing permeability of the skin should theoretically make it possible to transport molecules from inside the body through the skin to outside the body for collection or monitoring, practicable methods have not been disclosed. U.S. Pat. No. 5,139,023 to Stanley et al. discloses an apparatus and method for noninvasive blood glucose monitoring. In this invention, chemical permeation enhancers are used to increase the permeability of mucosal tissue or skin to glucose. Glucose then passively diffuses through the mucosal tissue or skin and is captured in a receiving medium. The amount of glucose in the receiving medium is measured and correlated to determine the blood glucose level. However, as taught in Stanley et al., this method is much more efficient when used on mucosal tissue, such as buccal tissue, which results in detectable amounts of glucose being collected in the receiving medium after a lag time of about 10-20 minutes. However, the method taught by Stanley et al. results in an extremely long lag time, ranging from 2 to 24 hours depending on the chemical enhancer composition used, before detectable amounts of glucose can be detected diffusing through human skin (heat-separated epidermis) in vitro. These long lag times may be attributed to the length of time required for the chemical permeation enhancers to passively diffuse through the skin and to enhance the permeability of the barrier stratum corneum, as well as the length of time required for the glucose to passively diffuse out through the skin. Thus, Stanley et al. clearly does not teach a method for transporting blood glucose or other analytes non-invasively through the skin in a manner that allows for rapid monitoring as is required for blood glucose monitoring, such as with diabetic patients, and for many other body analytes, for example blood electrolytes.
In view of the foregoing problems and/or deficiencies, the development of a method for safely enhancing the permeability of the skin for noninvasive monitoring of body analytes in a more rapid time frame would be a significant advancement in the art. It would be another significant advancement in the art to provide a portable device for non-invasively enhancing the permeability of the skin to analytes inside the body, providing an energy (ultrasound) source to speed and facilitate transport of the analyte, non-invasively collecting the analyte in a collection device such as a pad, analyzing (either on an existing analyzer or on an analyzer specific for this system) and monitoring those analytes, and accurately and reproducibly calculating the concentrations of those analytes inside the body at the site and time of collection.